Development of electrostatic image and apparatus therefor



Aug. 25, 1959 R; w. GUN DLACH 2,901,374

DEVELOPMENT OF ELECTROSTATIC IMAGE AND APPARATUS THEREFOR Filed May 4, 1955 I 3 Sheets-Sheet 1 2 ////////////A? IO j n :5 2 IO/ 4 H FIG. 2

POSITIVE HIGH VOLTAGE POWER SUPPLY POSITIVE H|GH VOLTAGE 23 w SUPPLY I ZI/ INVENTOR. ROBERT W. GUNDLACH H6. 4

BY n I EMA. 'ir

ATTORNEY Aug. 25, 1959 R. w. GUNDLACH DEVELOPMENT OF ELECTROSTATIC IMAGE AND APPARATUS THEREFOR 3 Sheets-Sheet Z Filed May 4, 1955 INVENTOR. ROBERT W. GUNDLACH FM A ATTORNEY Aug. 25, 1959 R. w. GUNDLACH 2,901,374

DEVELOPMENT OF ELECTROSTATIC IMAGE AND APPARATUS THEREFOR Filed May 4, 1955 3 Sheets-Sheet 3- INVENTOR. ROBERT w. GUNDLACH ATTORNEY United States Patent DEVELOPMENT OF ELECTROSTATIC IMAGE AND APPARATUS THEREFOR Robert W. Gundlach, Spencerport, N.Y., assignor, by

mesne' assignments, to The Battelle Development Corporation, Columbus, Ohio, a corporation of Delaware Application May 4, 1955, Serial No. 505,963

22 Claims. (Cl. 117- 175) This invention relates in general to a method and apparatus for development of an electrostatic image.

In Xerography it is usual to form an electrostatic image corresponding to a pattern of light and shadow to be recorded and to develop'the image or make it visible by deposition of electroscopic material thereon. Most simply and directly, a photoconduetive insulating layer is electrically charged and exposed to a light pattern, whereupon the charge is selectively dissipated to yield an electric charge pattern on its surface, this charge pattern generally being called an electrostatic latent image. In order to utilize this latent image in the production of a visible print, it ultimatelyis desired to deposit visible material in conformity with the image. One method of accomplishing this result is disclosed in Wise Patent 2,618,552, in which a two-component developer mixture is'cascaded or rolled across an image-bearing surface to deposit powder particles in conformity with the image while other systems, such as those disclosed in Carlson U.'S. 2,297,691, employ sprayed dust particles or other means or methods for deposition on the electrostatic latent image. These various systems of development have their relative merits and disadvantages. For example, a cascading system is quick and easy and is well adapted to the formation of dense black images of high contrast. However, it gives rise to problems in maintenance of proper relative proportions of the two components during prolonged operation and gives rise to many mechanical problems in feeding the two-component developer to the image-bearing surface.

Now, in accordance with the present invention, there are provided means, methods and mechanisms for the development of an electrostatic latent image whereby powder particles or the like are carried past an imagebearing surface by a support layer, such as a well, sheet, or the like, and are deposited directly on a printing material in conformity with said image to yield a print of excellent quality. The invention is particularly well suited to xerography in that it is inexpensive, convenient, and well adapted to automatic operation.

The nature of the present invention having been set forth, there will now bepresented a more detailed description, in illustration but not limitation, of the invention in the following specification and drawings in which:

Fig. l is a diagrammatic side cross section of an insulating surface supporting an electrostatic latent image;

Fig. 2 is a diagrammatic side cross section of developing mechanism for developing the image illustrated in Fig. 1;

Fig. 3 is a diagrammatic side cross section of an imagebearing member and printing member immediately after development has taken place;

Fig. 4 is a diagrammatic View of an automatic xerographic machine according to the present invention;

Fig. 5 is a diagramatic view of an automatic xerographic machine according to anotherembodiment of the present invention;

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Fig. 6 is a diagrammatic view of an automatic xerographic machine according to still another embodiment of the present invention;

Fig. 7 is a schematic diagram of the AC high voltage source shown in Fig. 2;

Fig. 8 is a diagrammatic view of an automatic xerographic machine according to still another embodiment of the present invention.

The present invention is particularly intended and adapted for the development of an electrostatic latent image, such as particularly a latent image of relatively fine detail composed of relative charged and uncharged areas on an insulating surface. Such as image may, for example, be an electrostatic latent image corresponding to a pictorial subject, such as a conventional photograph, snapshot, or the like, or may be an electrostatic latent image corresponding to a document, manuscript, or other representation of information to be reproduced. Images of this sort and other images requiring an optical resolution in the order of about ten lines per millimeter or better form the usual subject of development according to conventional xerographic systems.

Illustrated in Fig. l is such an image system, presented diagrammatically, comprising a xerographic member, generally designated 10, consisting of a conductive backing member 11, and an insulating layer 12 thereover which insulating layer desirably may be a photoconductive insulator. On the surface of the insulating layer 12 is an electrostatic image designated by the plus marks 14 at the surface of this layer. It is to be understood that the electrostatic latent image may be of either posi tive or negative polarity and may consist of gradations of potential or charge or may be, as illustrated in the figure, areas of charge interspaced by areas of substantially no charge. The particular image illustrated in Fig. 1 may be regarded as a line copy image consisting of representative areas of charge and absence of charge which desirably can be printed as areas of black on a white surface.

In Fig. 2 is shown diagrammatically developing means and apparatus suitable for developing the image illustrated in Fig. 1. Thus, for example, there is shown an insulating layer 12 having electrostatic image areas 14 on one surface thereof. This insulating layer may be either an insulating material, such as polystyrene, polyethylene terephthalate, or the like, or a photoconductive insulating material as described below in connection with Fig. 4. On the opposite side of the insulating layer 12 is a conductive layer 11 which is grounded. Positioned on top of the insulating layer 12 is a second insulating layer 15 bearing on the side away from the electrostatic image areas 14 a uniform coating of finely-divided powder particles 17 electrostatically adhering to the surface of the insulating layer 15. These powder particles may be any suitable xerographic toner material, this being generally a finely-divided powder material which is visible against a printing surface such as a surface of paper or the like and which is electrostatically attractable to an electrostatic latent image. It may, for example, consist of finelydivided pigment particles or finely-divided compositions of colored or pigmented resins or the like such as are well known to the art of xerography. Particularly satisfactory is charcoal and compositions such as described in US. 2,659,670 to Harold E. Copley. Positioned on top of the powder particles 17 and in contact therewith is a printing surface such as a surface of paper or the like 16 termed a printing member. The configuration of the separate layers in Fig. 2 has been exaggerated to more clearly depict the relationship of the layers. In actual construction the insulating layer 12 bearing the electrostatic image 14 is coated on the conductive backing member 11. The insulating layer 15 bearing the finely-divided powder material 17 is contacted with the insulating layer 12 by resting the layer 15 so that its Weight is supported solely by the Xerographic member while the printing member 16 is similarly resting in physical contact on top of the whole assembly. A corona electrode 39 connected to a source of high voltage A.C. 31 then moved across the top of the printing member 16 spraying the entire area with Zero bias. This has the same effect as pressing a conductive grounded electrode against this side of the printing member 16.

Where exposure has not resulted in the complete removal of charge from the areas that should appear white, the corona 39 rather than spraying the back of printing member 16 with zero bias may desirably spray the back of printing member 16 with a charge equal in potential and polarity to the lowest charge remaining on the image or a few volts, say about 10 volts, above this potential. This will minimize deposition of powder: in areas that should appear white.

Where the image to be developed consists of a con tinous tone image having areas of high charge, areas of intermediate charge, and areas of low charge, development may desirably take place in steps caused by passing corona 30 more than once over printing means 16. Thus, the corona Bil may desirably spray the back of printing member 16 with a potential equal in magnitude and polarity to that of the intermediate potential areas on the image. Only those areas of the image having a potential greater than that sprayed on the printing member 16 will transfer powder 17 from the insulating sheet at this pass of the corona 30. The corona 3b is then passed a second time over the back of printing member 16 to spray the back thereof with, say, the lowest potential remaining on the image. On this pass of the corona 30 those image areas having a potential between the lowest potential on the plate and the intermediate potentials on the plate will now develop for the first time while those areas having a potential between the intermediate potential areas and the high potential areas will develop for the second time. Thus, the use of multiple passes with different voltages sprayed on the printing member 16 makes possible a Wide variation of tone control in the resulting developed image transferred to the printing member 16. Rather than using a corona electrode, as shown, a conductive roller connected to a suitable source of variable D.C. may be run over the back of printing member 16.

Fig. 3 is a diagrammatic illustration of a mechanism of a development process illustrating the image-bearing surface 12, the printing member 16, and the powderbearing surface 15 after development of the electrostatic image has been accomplished. It is observed at this stage that a portion of the powder particles 17 originally residing on the insulating layer 15 has been transferred to form an image body 1% on the printing member 16. It should also be noted that the electrostatic image 14 remains unimpaired on the insulating surface 12' and may be subjected to another development cycle with a fresh supply of printing material and powder-bearing insulating layer. By this means, multiple reproductions may be obtained from a single electrostatic image. In general, the powder particles 17 may be regarded as being similarly charged with respect to the polarity of charge on the image-bearing surface so that these powder particles are repelled by the electrostatic charge of the image areas. The printing member 16 being grounded by the action of the corona as, the electrostatic charges of the image 14 induce charges of similar magnitude but opposite polarity on the printing surface 16, which serves to attract the powder particles 17, while at the same time these powder particles are repelled from the image areas of the layer 12. As a result of these forces, powder particles are attracted to the printing member 16 to form image areas thereon and are held in those areas by electrostatic attraction.

Fig. 4 illustrates a specific embodiment of a development member according to the present invention, wherein the Xerographic member, instead of a planar plate as in the assemblies designated 10 in Figs. 1, 2 and 3, is constructed in the form of a cylindrical drum designated 13. Such a machine is eminently adapted for continuous operation. As shown, the Xerographic member 13 consists of a layer of photoconductive insulating material 12 coated on the surface of a cylindrical drum consisting of a conductive material 11. This conductive backing portion 11 of the drum is desirably grounded, as shown. Alternatively, where exposure has not resulted in the complete removal of charge from the areas that should appear White, the conductive electrode may desirably be placed at the lowest potential remaining on the image or a few volts, say about 10 volts, above this potential. The conductive portion of the drum may consist of any conductive material having the desired strength and other constructional characteristics. Desirably, the drum is constructed of aluminum, although other materials such as Zinc, brass, magnesium, iron, tin, etc. may be used. The photoconductive insulating layer desirably consists of selenium, although it is understood that any such material may be used such as sulfur, anthracene, selenium-tellurium mixtures, zinc oxide, various phosphors, titanium dioxide, and other photoconductive materials, either as continuous layers or as discrete particles in a resinous binder. In the apparatus shown, the photoconductive insulating layer consisted of selenium. In this case, the layer is desirably sensitized by charging to a positive potential. In the case of materials such as anthracene and sulfur, the photoconductive insulating layer is desirably charged to a negative potential.

As shown, the charging is done by a corona electrode 20 which is connected to a suitable source of positive high voltage 21. Exposure means, as aperture 29, to place a pattern of light and shadow to be recorded on the photoconductive insulating layer 12 are positioned after the corona electrode 2%) in the direction of motion of the drum. Rolls 22 are positioned to feed in contact with layer 12 a supply of an insulating material 15 coated on the side opposite from layer 12 with a uniform layer of finely-divided material, the material loosely adhering to the insulating layer 15 by means of electrostatic attraction. Positioned opposite from the Xerographic member 13 is a cylindrical drum 24 of yielding conductive material, such as a conductive rubber, so positioned as to contact the Xerographic member 13 tangentially at the point of contact of the powder-carrying insulating layer 15. Reels 23 lead a supply of print-receiving material, as paper, 26 through the point of tangential contact and so positioned that the print-receiving material is between the powder-carrying surface 15 and the conductive roller 24. As suiable fixing means 25 a fusing device is positioned ahead of the take-up reel so that the print-receiving material 26 passes through the fixing means 25 before being received by the take-up reel 23. Corona 27 connected to a suitable source of positive high voltage 28 is positioned adjacent to the powder-carrying side of the insulating material 15 prior to the passage of the insulating layer into the Zone of contact with the Xerographic member 13. The Xerographic member 13 and reels 22 and 23 are all connected to drive means as motors, springs, etc. Roller 24 may also be connected to such drive means, if desired, but preferably is left free to rotate on its longitudinal axis.

In operation, the positive high voltage power supplies 21 and 28 are turned on, the drive means are activated to drive reels 22 and Xerographic member 13 counterclockwise, and drive means are similarly activated to drive reels 2.3 clockwise. Xerographic member 13 is shielded from light with the exception of aperture 29. The photoconductive insulating layer 12 passes under the corona 20, where it is sprayed with positive high voltage thereby causing a high positive charge to be placed on the photoconductive insulating layer. This then passes under aperture29 where it is exposed to a pattern of light and-dark, thereby causing the light areasof the selenium surface to be rendered conductive and-creating thereby a charge pattern on the photoconductive insulating layer 12 corresponding'to the pattern of light and shadow impressed at the exposure aperture 29.

The drum is then rotated into the point of-tangential contact with the roller 24. Reels 22 lead a supply of insulating material 15- through this zone of tangential contact. The side of the layer 15 opposite from the photoconductive insulating layer 12 is coated with a uniform layer of finely-divided material adhering to the insulating layer 15 by means of electrostatic attraction. Prior to the entry of this layer into the point of tangential contact, the layer of finely-divided materials is sprayed with positive corona discharge from corona 27. Reels 23 feed a supply of printing material, such as paper, through the zoneof tangential contact between the insulating layer 15 and the conductive roller 24. As the layer of insulating material 15 and, print-receiving material 26 pass through the tangential zone, transfer of powder from the layer 15 to the printing material 26 occurs in conformity with the electrostatic image borne on the photoconducti e insulating layer 12, precisely as shown in Fig. 3. After .leavingthe zone of tangential contact, the printreceiving material '26, now bearing an image of finelydivided material adhering loosely to the printing means by means of electrostatic attraction, passes through a fusing device 25 wherein the image is fused to the printing material and firmly bound thereto. This fusing device 25 may be of heat, as shown, which causes the particles of finely-divided material to soften and melt and thereby fuse to the printing material or by means of solvent vapor which renders the particles of developer material tacky, also causing them to adhere closely to the printing material. After leaving the fusing zone 25, the printing material is then wound on the take-up reel 23.

After leaving the zone of tangential contact, the photoconductive insulating layer once again passes under corona 20 to be recharged and the cycle is repeated until the printing material 26 and the insulating material 15 are exhausted. Preferably, supply reel 22 has interleaved between the layers of insulating material a shielding material, as paper, preventing toner particles from transferring from the top of the one layer whereon they are coated to the bottom of the layer immediately above. In this Way, the side of layer 15 presented to the photoconductive insulating layer 12 is free fromtoner particles. This eliminates the necessity of a cleaning station on the xerograpliic member 13. Failure to supply such interleaving necessitates the addition of a cleaning station prior to corona station '20 on the Xerographic drum.

Fig. 5 issimilar to Fig. 4 with the addition of a continuous belt of insulating material 34 positioned on rollers 32 and 33 and xerographic drum 13 so as to pass between xerographic drtun 13 and the layer of insulating material'15 bearing the toner particles. Layer 34 is a continuous film so that solid particles evenof finelydivided toner cannot pass through and, in addition, has sufiicient resistivity to prevent lateral conductivity with consequent blurring of the image on the photoconductive insulating layer. The use of such a continuous belt and separate rollers 32 and 33 eliminates the necessity for interleaving of the insulating layer 15 on reels 22, as well as the alternative in Fig. 4 of supplying a cleaning station on the xerographic drunr 13' prior to the corona 20. The manner of operation of the apparatus in Fig. 5 is similar to that of Fig. 4 with the. exception that, if desired, drive means may be supplied for reels 32 and 33. Alternatively, these reels may also be merely free to rotate on their longitudinal axes, as is the case in the preferred method of operation of roller 24.

In Fig. 6the apparatus described is similar to Fig. 5 with the exception that reels 22 supplying an insulating film coated with toner particles has been omitted and insulating layer 34 supplies the same function. In this case, the bottom reel '33 is positioned so that film 34 passes into a box 40 filled with toner particles 44. The mate rial of layer 34is selected of the correct tniboelectric rela tionship to that. of the particles of. developer 44 that in passing through thebox 40 around reel 33, the belt and the powder particles in contact therewith are charged by triboelectric'conta'ct and the particles consequently adhere to the belt throughelectrostatic attraction. If desired, after leaving the box 40, the belt may pass under corona means 27'connected'to a source of positive high voltage 28 and. thence around'reel 32 and into the point of tangential contact between the xerographic drum 13 and the conductive roller 24. With this exception, the method of operation of the apparatus is as described in Fig. 4. Drive means as a motor, springs, etc. drive reels 32 and 33'counterclockwise and a motor 42 is also supplied to drive an eccentric 41 by means 43. The eccentric 41 is sopositioned that on rotation, it causes the box 40 to vibrate, thereby preventing the developer particles 44 from packing and assuring close contact between the developer particles 44 and the web 34.

Fig. 8' is similar to. Fig. 4,, with the exception that the positionofreels 2'2 and 23 and, therefore, of the powder-carrying insulating layer 15 and the printing means26, have been reversed. This necessitates a change in the polarityoffthe high voltage supply. to the powder particles coated; onthe insulating layer 15. In this instance, corona electrode 55, is connected to a negative high voltage supply 54; Transfer within the point of tangential contact between roller 24 and xerographic drum 13 proceeds as: setforth in Fig. 4 but in the reverse direction.

Fig. 7 shows the source,;of Fig. 2, in more detail. Here .asource of 1l0-volt 60-cyole A.C. 51 is supplied to atransforrner 50 wherein the voltage is, multiplied. to the desired value for corona discharge, say 6,000volts, and the high side of this transformer connected. tocorona wire 30. A shield 53 is placed about corona, wire 30 and is grounded, as shown. A condenser52, is-connectedbetween the low side of the transformer 50, and ground. If desired, this apparatus may be adapted .as the source ofpositive high voltage 21 and/or 22 of Figs. .4, 5, 6 and 8 or negative high voltage 54 of Fig. 8.

Whenit is to be. used in this manner, the condenser 52, insteadof being grounded as shown in Fig. 7, is connected toa suitable source of DC, a battery, of the polarity and potential equalto that which it is. desired to place on the material to be charged, such as the photoconductiveinsulating layer 12 of xerographic member 13 or the finely-divided powder particles on the insulating layer 15. The other, terminal of the DC. source not connected .to thecondenser 52 is grounded. The corona shield 53 is then connected between the condenser 52 and the source of.D.C. potential thereby placing on the shield 53-the potential and, polarity to which it is desired to chargethe particular material. A suitable DC. potential, for instance, would he, say, 500 volts. The charge delrvered to the corona electrode 30 by transformer 50 is that sufficient to. obtain the .desired rate of corona current. At atmospheric pressure 6000 volts has been found tobe suflicient WlIlLSOO volts on the shield 53 to give adequate charging rates. As the corona current is supplied bythe A.C. source, there is virtually no drain on the DC. source impressed on corona shield 53. In addition, this arrangementhas been found to virtually eliminatethe possibilityof overcharging.

It is to be understood that the invention herein described may be suitablymodified as will be apparent to those skilled in 'the art and that the disclosure herein A.C. high voltage 31 of should be taken in illustration and not in limitation of the scope of the invention.

No matter which embodiment of the invention is used, many advantages will necessarily result. Thus, the instant process makes it possible to completely eliminate the necessity for flowing development powders across the image-bearing surface with consequent abrasion of this surface. In addition, it makes possible the elimination of the necessity of cleaning the xerographic member after development either by flowing a granular cleaner across the surface or rubbing with a suitable brush or fur-bearing member. By eliminating this step it is possible to effect a considerable reduction in the overall time needed to develop a visible image. The elimination of the cleaning step, taken together with the elimination of flowing the developer over the image surface, considerably reduces abrasion and wear of the photoconductive insulating surface with consequent extension of the life of this part of the xerographic machine and makes possible much more compact apparatus. In addition to greater freedom in design the reduction in size of the xerographic drums makes possible a definite cost saving. Furthermore, the presence of grounding means, such as the conductive roller 24, or the use of zero bias on the back of printing means 16 in Fig. 2, which means are essential in the process of the instant invention, constitutes a development electrode. The improvement in image quality attendant on the development electrode is thus available for a development system other than powder cloud development. The process is readily adapted for continuous machines. Finally, since the latent image is not impaired, recycling with charging and exposure shutdown would make multiple copying possible.

What is claimed is:

1. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, contacting one side of said sandwich with an insulating surface hearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

2. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising positioning on a con ductive backing member an insulating surface bearing the electrostatic latent image, forming a deposit of finelydivided electrostatically charged powder material on one surface of an insulating sheet, bringing the side of said sheet opposite from that on which the powder material was deposited into contact with said insulating surface thereby forming a development zone while contacting a printing sheet with the opposite side of said insulating sheet, applying a voltage at the side of said printing sheet not in contact with the said insulating sheet to cause the said powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

3. A method of developing an electrostatic latent image comp-rising areas of varying charge on an insulating surface, said process comprising forming a deposit of finelydivided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form, a sandwich, contacting one side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in confromity with said electrostatic latent image, removing said printing sheet from the said development zone and fusing the finely-divided powder material to the said printing sheet.

4. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, contacting one side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in confromity with said electrostatic latent image, removing said printing sheet from said development zone and applying heat to the said printing sheet whereby the finely-divided powder material is tackified and adheres to the said printing sheet in conformity with said electrostatic latent image.

5. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, contacting one side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image, removing said printing sheet from said development Zone and passing the said printing sheet through the vapors of a solvent for the said powder material whereby the said powder material is rendered tacky and adheres to the said printing sheet in conformity with said electrostatic latent image.

6. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, contacting the insulating sheet side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in conformity with electrostatic latent image.

7. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided powder material on one surface of an insulating sheet, electrically charging said powder material to the same polarity as that of the electrostatic latent image to be developed, bringing a printing sheet into contact with the charged powder-bearing side of said insulating sheet to thereby form a sandwich, contacting the insulating sheet side of said sandwich with an insulating surface bearing the electrostatic latent image, said insulating surface being positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in con formity with said electrostatic latent image.

8. A method of developing an electrostatic latent image comprising areas of varying charge on an nsulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulatingsheet to thereby form a sandwich, contacting the printing sheet side of said sandwich with an insulating surface bearing the electrostatic latent image, said insulating surface being positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

9. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided powder material on one surface of an insulating. sheet, charging said powder material to a polarity opposite to that of the electrostatic latent image to be developed, bringing a printing sheet into contact with the charged powder-bearing side of said insulating sheet to thereby form a sandwich, contacting the printing sheet side of Said sandwich with an insulating surface bearing'the electrostatic latent image, said insulating surface being. positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

10. A method of developing an electrostatic image comprising areas of varying charge on an insulating surface which is positioned on a conductive backing member, said process comprising continuously moving said insulating surface through a development zone, contacting said insulating surface with an insulating sheet hearing a deposit of finely-divided electrostatically charged powder material on the side not in contact with the said insulatingsurface while contacting a continuous supply of printing material with the powder-bearing surface of the said insulating sheet in the development zone and simultaneously applying a voltage to the opposite side of said. printing sheet from said insulating surface to cause the powder material to deposit on said printing sheet in conformity with said electrostatic latent image, continuously removing said printing sheet now bearing a powder image from said development zone, fusing the said powder to the said printing sheet.

11. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on a porous insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, positioning an insulating surface bearing an electrostatic latent image on a conductive backing member, placing a continuous film of insulating material on said insulating surface in a development zone, contacting the insulating sheet side of said sandwich with insulating film in the development zone, applying a voltage to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

12. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating' surface, said process comprising forming a deposit of finely-dividedelectrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of saidinsulating sheet to thereby form a sandwich, contacting one side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, and applying a voltage to the opposite side of said sandwich'from said insulating surface by a corona discharge from a corona electrode connected to an A.C. high voltage source to cause powder material to deposit on said printing sheet in conformity with said-electrostatic latent image.

13. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, contacting one side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, positioning a conductive electrode on the opposite side of said sandwich from said insulating surface, and applying a voltage between the conductive electrode and the conductive backing member to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

14. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing a printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, contacting one side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, placing a conductive electrode on the opposite side of said sandwich from said insulating surface and applying ground potential to both the conductive electrode and the conductive backing member to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

15. A method of developing an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising forming a deposit of finely-divided electrostatically charged powder material on one surface of an insulating sheet, bringing the printing sheet into contact with the powder-bearing side of said insulating sheet to thereby form a sandwich, contacting one side of said sandwich with an insulating surface bearing an electrostatic latent image, said insulating surface being positioned on a conductive backing member, applying a ground potential to said conductive backing member, applying a voltage equal in charge and polarity to the lowest voltage appearing on the said insulating surface to the opposite side of said sandwich from said insulating surface to cause powder material to deposit on said printing sheet in conformity with said electrostatic latent image.

16. A method of producing multiple visible copies of an electrostatic latent image comprising areas of varying charge on an insulating surface, said process comprising positioning the insulating surface bearing the electrostatic latent image on a conductive backing member, continuously contacting one surface of a sheet of insulating material with said insulating surface, the opposite side of said insulating sheet bearing a deposit of finely-divided electrostatically charged powder material, contacting a sheet of printing material with the powderbearing surface of said insulating sheet in contact with said insulating layer and applying a voltage from ground potential to the lowest potential on the insulating surface and of the same polarity as on the insulating surface to the side of the printing material not in contact with the insulating sheet to cause the powder material to deposit on said printing sheet in conformity with said electrostatic latent image, removing the printing sheet in contact with the insulating sheet and fusing the powder image thereon.

17. Apparatus for developing an electrostatic latent 11 image comprising areas of varying charge on an insu lating surface, said apparatus comprising an insulating surface positioned on a conductive backing member to form the image-bearing member, means to move said image-bearing member past a development station, an insulating sheet positioned at said development station with one surface in firm contact with the image-bearing member at said station and the opposite surface bearing a deposit of finely-divided powder material, drive means to move said insulating sheet through said development station and in contact with the image-bearing member in said development station, a printing sheet positioned at said development station with one surface in firm contact with the powder-bearing surface of the said insulating sheet, drive means to move said printing sheet through said development station and in contact with the powderbearing surface of the insulating sheet, and means for applying a voltage to the opposite side of the said printing sheet to cause powder material to deposit on said printing sheet.

18. Apparatus for developing an electrostatic latent image comprising areas of varying charge on a photoconductive insulating surface, said apparatus comprising a photoconductive insulating layer coated on a conductive backing member to form an image-bearing member in the shape of a cylindrical drum, means adapted to move said drum on its longitudinal axis past a charging station and exposure station and a development station, charging means positioned at said charging station to sensitize said photoconductive insulating layer, exposure means at said exposure station to expose the said photoconductive insulating layer to a pattern of light and shadow to be recorded, supply means to feed an insulating sheet through said development station in firm contact with said drum to a take-up means, supply means to feed a printing sheet through said development station and in firm contact with the insulating sheet at the development station to take-up means, fusing means between the development station and the printing sheet take-up means and a drum-shaped conductive electrode adapted to rotate on its longitudinal axis and positioned in yielding contact with the xerographic drum at the development station and means to apply a D.C. voltage to the said conductive electrode.

19. Apparatus for developing an electrostatic latent image comprising areas of varying charge on a photoconductive insulating surface, said apparatus comprising a photoconductive insulating surface positioned on a conductive backing member in the form of a cylindrical drum, means adapted to move said image-bearing member past a development station, an insulating sheet positioned at said development station with one surface adapted to be brought into firm contact with the imagebearing member at said station and the opposite surface bearing a deposit of finely-divided powder material, supply means for said insulating sheet and take-up means for said insulating sheet, drive means adapted to move said insulating sheet from said supply means through said development station in contact with the image-bearing member thereon to said take-up means, charging means between said supply means and said development station to apply an electric charge to the finely-divided powder material on said insulating sheet, a printing sheet positioned at said development station with one surface adapted to be brought into firm contact with the powderbearing surface of the said insulating sheet, supply means for said printing sheet and take-up means for said printing sheet, drive means to move said printing sheet from said supply means through said development station in contact with the powder-bearing surface of the insulating sheet through a fusing device to said take-up means, a cylindrical conductive electrode adapted to rotate on its longitudinal axis and adapted to be brought into firm contact with the printing sheet at the development station, and means for applying a voltage between the said conductive electrode and the conductive backing member of the said insulating surface.

20. Apparatus for developing an electrostatic latent image comprising areas of varying charge on a photoconductive insulating surface, said apparatus comprising a photoconductive insulating surface positioned on a conductive backing member in the form of a cylindrical drum as an image-bearing member, means adapted to move said image-bearing member past a development station, an insulating sheet in the form of a continuous belt positioned at said development station and adapted to be brought into firm contact with the image-bearing member at said station, an insulating web positioned at said development station with one surface to be brought into firm contact with the insulating sheet at said station and the opposite surface bearing a deposit of finely-divided powder material, supply means for said insulating Web and take-up means for said insulating means, drive means adapted to move said insulating web from said supply means through said development station in contact with the insulating sheet therein to said take-up means, a printing sheet positioned at said development station with one surface adapted to be brought into firm contact with the powder-bearing surface of the said insulating web, supply means and take-up means for said printing sheet, drive means to move said printing sheet from said supply means through said development station in contact with the powder-bearing surface of the insulating web therein through a fusing means to said take-up means, a conductive electrode in the shape of a cylindrical drum adapted to revolve on its longitudinal axis and adapted to be brought into firm contact with the printing sheet at said development station, and means to apply a voltage between said conductive electrode and said conductive backing member.

21. Apparatus for developing an electrostatic latent image comprising areas of varying charge on a photoconductive insulating surface, said apparatus comprising a photoconductive insulating surface positioned on a conductive backing member to form an image-bearing member in the shape of a cylindrical drum, means adapted to rotate said image-bearing member on its longitudinal axis past a development station, an insulating sheet in the form of a continuous belt positioned at said development station with one surface adapted to be brought into firm contact with the image-bearing member at said station, a supply of finely-divided powder material in a container, means to vibrate the container, means to contact with the said powder material in said box only that side of said insulating sheet which does not contact said photoconductive insulating layer, the insulating sheet and powder material being so chosen from a triboelectric series that the frictional contact of said insulating sheet moving through the said powder supply charges the said powder and belt causing a supply a powder to adhere to the belt by electrostatic attraction, drive means adapted to move said insulating sheet from said powder supply to said development station and in contact with the imagebearing member in said development station, a printing sheet positioned at said development station with one surface adapted to be brought into firm contact with the powder-bearing surface of the said insulating sheet, supply means and take-up means for said printing sheet, drive means adapted to move said printing sheet from said supply means through said development station and in contact with the powder-bearing surface of the insulating sheet therein then through a fusing device to the take-up means, a conductive electrode adapted to be brought into firm contact with the printing sheet at said development station, and means for applying a voltage between said conductive electrode and said conductive backing member.

22. Apparatus for developing an electrostatic latent image comprising areas of varying charge on a photoconductive insulating surface, said apparatus comprising a photoconductive insulating surface positioned on a conductive backing member to form an image-bearing member, means adapted to move said image-bearing member past a development station, a printing sheet positioned at said development station with one surface adapted to be brought into firm contact with the image-bearing member at said station, drive means adapted to move said printing sheet through said development station and in contact with the image-bearing member in said development station, an insulating sheet positioned at said development station with one surface adapted to be brought into firm contact with the printing sheet at said station, said surface bearing a deposit of finely-divided powder material, drive means adapted to move said insulating sheet through said development station and in contact with the printing sheet in said development station, charging means to apply an electric charge to the powder material of a polarity opposite to that of the electrostatic latent image, said charge being applied to the powder material at a point ahead of its entry into the development station, a conductive electrode positioned at said development station and adapted to be brought into firm contact with the insulating sheet at said development station and means for applying a voltage between said development electrode and said conductive backing member to cause the powder material to deposit on said printing sheet in conformity with the electrostatic latent image.

References Cited in the file of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,297,691 Carlson Oct. 6, 1942 2,633,796 Pethick Apr. 7, 1953 2,694,416 Butterfield Nov. 2, 1954 2,752,833 Jacob July 3, 1956 2,758,524 Sugarman Aug. 14, 1956 2,758,525 Yeates Aug. 14, 1956 2,758,939 Sugarman Aug. 14, 1956 2,803,177 Lowrie Aug. 20, 1957 2,808,328 Jacob Oct. 1, 1957 

1. A METHOD OF DEVELOPING AN ELECTROSTATIC LATENT IMAGE COMPRISING AREAS OF VARYING CHARGE ON AN INSULATING SURFACE, SAID PROCESS COMPRISING FORMING A DEPOSIT OF FINELY-DIVIDED ELECTROSTATICALLY CHARGED POWDER MATERIAL ON ONE SURFACE OF AN INSULATING SHEET, BRINGING A PRINTING SHEET UNTO CONTACT WITH THE POWDER-BEARING SIDE OF SAID INSULATING SHEET TO THEREBY FORM A SANDWICH, CONTACTING ONE SIDE OF SAID SANDWICH WITH AN INSULATING SURFACE BEAR- 